Valves for control of compressed air to brake cylinders in railroad braking systems generally have three basic positions:
(1) A position in which air is admitted from a supply of compressed air to the brake cylinder, this compressed air serving to apply the brakes. PA1 (2) A position in which air is exhausted from the brake cylinder, which causes the brakes to be released. PA1 (3) A position, referred to as the lap position, in which air is neither supplied to, nor exhausted from the brake cylinder. This position is used to maintain the brake application forces approximately constant.
In the past, this function has been provided by a valve having a linear displacement spool. The spool is positioned inside a cylindrical housing and is moved axially to change from one valve position to another. An example of this is U.S. Pat. No. 4,773,447 entitled ELECTROMAGNETIC VALVE, invented by Asaji Imanaka and Mitsuhiro Ikeda.
Valves of this type have the disadvantage that very precise machining is required, particularly in the bores where sealing is provided. Some of these valves require dynamic O-rings, which are subject to rubbing each time the valve position is changed. Some of these valves employ diaphragms. These valves require precise guiding, and various parts are needed for clamping the diaphragms.
Valves of the linear displacement type also have a large number of parts which must be assembled, and which must be disassembled for overhaul.